Underwater inspection apparatus and method

ABSTRACT

A holographic recording and reproducing system for recording holographic images of an object positioned in a first medium and replaying said images in a second medium incorporates means for enhancing the relative sensitivity of the system to radiation capable of producing said holographic images.

This invention relates to holography and, in particular, to methods ofand apparatus for the holographic inspection of underwater objects suchas pipelines.

The recovery of oil and gas has presented a significant challenge to theoffshore industry as regards routine inspection and maintenance ofsubsea installations. As drilling now proceeds to even greater depthsthe problems encountered increase and more emphasis is now being placedon remote, rather than diver held, techniques of inspection. Visualinspection is extensively utilised with the major part of this beingcarried out using conventional photography, stereo photography andclosed-circuit television. These methods all, however, suffer severaldrawbacks. Conventional photography produces two-dimensional images ofmoderate resolution but loses parallax information and, particularly inclose-up, suffers from perspective distortion and limited depth offield. Stereo-photography improves this situation by producing athree-dimensional image from two fixed viewpoints: parallax informationis still lost. Furthermore, if precise dimensional measurements arerequired, sophisticated photogrammetric techniques are necessary withlimited resolution as yet occupancy. Television systems while providingreal-time operation are essentially low resolution techniques.

Holography, by comparison, suffers from none of these limitations andgives the observer an infinity of viewpoints from which the view thescene. It is the ability of holography to reproduce, remote from theoriginal scene, a full size three-dimensional image possessing highresolution and low in optical aberrations which make it a potentiallypowerful method of visual inspection. Applications which can beenvisaged include general archiving, measurement of corrosion pittingand cracking, examination and measurement of damage sites, structuralprofiling and examination of marine growth. In all such applications therequired end product is usually a high resolution hologram of aparticular scene of interest. From this hologram, inspection can beperformed directly on an image reconstructed in real space.

Holographic visual inspection or "hologrammetry" as it has now come tobe known, is becoming increasingly important as a means of making highresolution dimensional measurements of engineering components andstructures. This is particularly true when the inspection site islocated in a hazardous environment or is an area where access isdifficult, such as encountered in the nuclear power generating industryor the offshore oil and gas industries.

The basis of holography as a means of high resolution visual inspectionis the holographic recording of the scene of interest with thesubsequent replay of the processed hologram in the real image mode ofreconstruction. Reconstruction of the real image produces an image whichis reversed left-to-right and back-to-front when viewed from the spacein front of the hologram. Such a representation of the image is known as"pseudoscopic".

In general, the holographic interference field is captured onphotographic film. Other media such as thermoplastic film, photochromicmaterials, non-linear optical crystals and dichromated gelatin may,alternatively, be used. Holographic film differs from the film used inordinary photography only in that the grains of silver halide are of theorder of a few nanometers across as compared to micrometers. Such filmis very insensitive to light but has the capacity to record the finedetail inherent in an interference field. Typical sensitivity is arounda few millijoules per square meter. The exposed film is chemicallyprocessed in a similar, but somewhat more elaborate, way to ordinaryfilm to render the holographic interference permanent.

For purposes of visual inspection, however, creation of the virtualimage is not the most suitable form of holographic reconstruction. It sohappens that if we turn the plate around and illuminate it from behindwith a wave which is the exact conjugate of the reference beam, aconjugate image will be located in real space in front of the plate. Theimage so created is optically identical to the original save that itappears to be reversed left to right and back to front. It is this real,pseudoscopic image which forms the basis of a method of visualinspection.

The utilisation of holography in visual inspection, relies on thecreation and optimisation of the real image of a scene or object. Thereal, or more correctly the conjugate image, is formed andreconstructed. A parallel reference beam is often used in recording thehologram, since reconstruction then is a simple case of turning the filmaround. If a diverging reference beam was used in recording, then aconverging beam of identical curvature would be needed inreconstruction.

Because the conjugate image is located in real space in front of theobserver, visual inspection can be carried out directly on this imageusing all the conventional tools of the trade, namely, measuringmicroscopes, photography and TV. Optical sections can be taken throughthe resulting reconstruction by merely placing a piece of film acrossthe image and recording directly, without the need for any lenses. Thisconcept is sometimes hard to accept without seeing it. An image isactually formed in space in front of the observer on which all opticaltests can be performed as if it were the original subject.

The usefulness of holography for accurate engineering measurement,whether this be in water or in air, is dependent on its ability toreproduce an exact image of the object which is low in opticalaberrations and possesses sufficient resolution to allow detailedmeasurements to be made. In practice, loss of resolution and aberrationscan occur at both recording and replay stages. There are severalrecognised factors which can give rise to optical aberrations and,ultimately, degrade image fidelity. Such factors include,

(a) distortion of the fringe pattern recorded in the emulsion as aresult of chemical processing,

(b) the optical quality of the emulsion substrate

(c) variations between reference and reconstruction wavefronts

(d) variations between reference and reconstruction wavelength, and

(e) the quality of the reconstruction beam.

The above factors affect the ability to produce optimum conditions forwavelength reconstruction but can, with reasonable precautions, becontrolled to a degree sufficient to produce high resolution images.

In accordance with the present invention there is provided a holographicrecording and reproducing system for recording holographic images of anobject positioned in a first medium and replaying said images in asecond medium characterised in that it includes gating meanssubstantially to reduce the effect on a photosensitive medium ofradiation other than radiation capable of producing said holographicimages.

The invention will be particularly described with reference to theaccompanying drawings in which:

FIG. 1 shows an optical arrangement for recording holograms,

FIG. 2 is a photograph of a hologram taken using the arrangement of FIG.1,

FIG. 3 is a diagrammatic representation of a cell used to produceinterferograms

FIG. 4 is an optical arrangement used with the cell of FIG. 3,

FIG. 5 is an interferogram produced using this arrangement,

FIG. 6 is a photograph taken from a real image reconstruction of thehologram of a revolving paddle,

FIG. 7 is a graph showing the variation in attenuation length withwavelength for distilled water,

FIG. 8 is a diagram of an experimental arrangement for measurement ofbeam scattering in water,

FIGS. 9 (a) and (b) show respectively polarisation ratio and totalirradiance against scattering angle,

FIG. 10 is an experimental arrangement for measurement of depolarisationat a rough surface,

FIGS. 11 (a) and (b) show respectively polarisation ratio and totalirradiance against scattering angle for a rough surface,

FIGS. 12 and 13 show photographs taken from different objects in water,

FIGS. 14 to 18 are explanatory ray diagrams,

FIGS. 19 (a) and (b) are diagrammatic arrangements of holographiccamera, and

FIG. 20 is an optical arrangement for reconstruction of holographicimages.

Referring now to the drawings, FIG. 1 shows an optical arrangement forthe recording of holograms. An argon ion laser 1 with a shutter 2 servesas radiation source. A variable beam splitter 3, separates the radiationinto recording and reference beams 4,5 respectively. The reference beampasses by way of mirror M1,M2,M3 through a microscope objective andpin-hole spatial filter 6 to produce a collimated beam 7 whichilluminates a holographic plate 8. The recording beam 5 passes by way ofmirror M4,M5 and microscope objective and pin-hole filter 9 toilluminate a moveable target 10 in a water tank 11. A real image 12 ofthe hologram is viewed in air by means of a travelling microscope 13.The whole arrangement is mounted on a pneumatically supported table 14to reduce vibrations and extraneous thermal effects.

The primary monochromatic aberrations to be found in any imaging systemare those of spherical aberration (S), chromatic aberration (C),astigmatic aberration (A), field curvature (F), and distortion (D). Therelative coefficients of aberration for each of the above can be givenin terms of the cartesian co-ordinates relating to object, reference andreconstruction wave positions with respect to the hologram plane.

It can be shown that the aberration coefficients, S, C. A, F and D canbe minimised if two conditions are met, namely, that

(a) the reference and reconstruction wavefronts are both located atinfinity, that is, they should be collimated, and,

(b) the wavelength of the reference wave must equal that of thereconstruction wave.

Correspondingly these conditions yield the fact that when reconstructedas above the lateral, longitudinal and angular magnifications of thereal image will all be equal to unity.

The above arguments apply to the paraxial region. For the non-paraxialcase more rigorous formulations apply. However, we can realisticallyassume that, if we meet the above conditions, then all aberrations willbe reduced to a minimum.

In real-image reconstruction, image resolution is limited, in theory,only by the quality of the reproduced hologram. The resolution of aholographic image is usually defined as the ability to distinguish twopoints on a hologram separated by distance r, given as,

    r=1.22λz/D                                          (1)

where λ is the reconstruction wavelength, z is the separation betweenhologram and reconstructed image and D is the effective aperture of thehologram. This equation is the standard relationship of optics whichdefines the resolving power of a lens. A lens and a hologram ofequivalent diameters will produce the same theoretical resolution.Because of its reduced susceptibility to optical aberrations, thehologram will produce the more highly resolved image.

The above equation is usually expressed as a resolving power (R) in linepairs per millimeter (1 p/mm) by reciprocating and dividing by 10³.Hence, we have

    R=10.sup.-3 /(1.22zλ/D)[1 p/mm]                     (2)

In holography the presence of speckle effects introduced by thecoherence of the light and the finite aperture of the viewing systeminfluences the resolving power. The speckle size sets the lower limit tothe resolution. In practice, R is reduced by a factor of 2 to 3 takeaccount of this.

The resolution obtained in this way assumes that we reconstruct with anexact conjugate of the reference beam, that the reconstructionwavelength matches that of the recording beam and that the emulsion isinfinitesimally thin. Conversely, the resolution of a virtual imagehologram is limited by the effective aperture of the viewing system.

Emulsion uniformity and substrate quality have a significant effect onthe quality of the reconstructed hologram. Standard holographic platessupplied by the major manufacturers often exhibit a departure fromflatness of up to a few hundred fringes. To improve the holographicimage from such plates the reconstructed area can be apertured to coverthat part of the plate exhibiting best flatness.

The quality of the recorded hologram depends to a large extent on thechoice of photographic emulsion and processing techniques. The choice ofemulsion falls between:

(a) Agfa 8E56HD and Ilford SP672

Both these films are sensitised for use with lasers operating in theblue-green region of the spectrum and feature fine grain withconsequently low scatter and high diffraction efficiency.

(b) Agfa 8E75HD and Ilford SP673

These emulsions are similar to those mentioned above but are sensitisedfor work in the red region of the spectrum.

(c) Agfa 10E56HD and Agfa 10E75HD

These emulsions possess significantly larger grain sizes than any of theother emulsions and consequently need less exposure to light. The largegrain size, however, reduces the resolving power of the film and theycan therefore not be recommended for high resolution holography.

Chemical processing of the recorded interference pattern is a crucialstep in the holographic procedure. Some of the factors which have to beconsidered include image brightness, image resolution, reconstructionwavelength, emulsion shrinkage and noise level.

Several processing procedures have been found to be suitable forprocessing of transmission holograms. These processes include:

(a) develop only

(b) develop and fix

(c) develop and bleach (rehalogenating)

(d) develop and bleach (reversal)

(e) develop, fix and bleach.

It has been established that for bright holograms on silver halide filmbleaching is desirable in to maximise the efficiency of the hologram.Some forms of bleaching, though, can result in non-uniform shrinkage ofthe emulsion, which will give rise to astigmatism in the reconstructedimage. A rehalogenating bleach process, in which the developed grains ofsilver are reconverted to silver bromide, has been shown to be the mostsuitable process for holography giving rise to a bright image with lowscatter. Because no silver is removed but is merely redistributedthrough the emulsion it is believed that this type of processing resultsin the dimensions of the emulsion remaining constant before and afterexposure.

A preferred method for processing holograms includes the followingsteps:

Pre-wash: 2 min in de-ionised water at 20° C.

Develop: 2 min in Pyrogallol at 20° C.

Wash: 2 min in de-ionised water at 20° C.

Bleach: 2 min in Ferric EDTA at 20° C.

Wash 1: 10 min in de-ionised water

Wash 2: 2 min in 50/50 de-ionised water and methanol

Wash 3: 2 min in 100% methanol

Tetenal Neofin Blue is prone to emulsion shrinkage and for more exactingwork it can be replaced by a pyrogallol based developer such asAgfa-Gevaert GP62, the formulation of which is given in Table 1.

                  TABLE 1                                                         ______________________________________                                        Part A            Part B                                                      ______________________________________                                        700 ml water      700 ml water                                                 15 g metol        60 g Na.sub.2 CO.sub.3                                      7 g pyrogallol   de-ionised water to 1000 ml                                  20 g Na.sub.2 SO.sub.3                                                        4 g KBr                                                                       2 g Na-EDTA                                                                  de-ionised water to 1000 ml                                                   ______________________________________                                    

Working solution made up as 1 part A+2 water+1 part B. The bleachformulation used was as in Table 2:

                  TABLE 2                                                         ______________________________________                                        700  ml     water                                                             50   g      potassium bromide [KBr]                                           1.5  g      boric acid                                                                    water to 1000 ml                                                  2    g      para-benzoquinone [PBQ] added just before use.. .                 ______________________________________                                                    .                                                             

An alternative bleach to the PBQ based one described above may be usedbecause of its less toxic properties. The formula of this bleach isgiven in Table 3:

                  TABLE 3                                                         ______________________________________                                        700      ml        de-ionised water                                           30       g         ferric sulphate [Fe.sub.2 (SO.sub.4).sub.3 ]               30       g         EDTA di-sodium salt                                        30       g         potassium bromide [KBr]                                    10       ml        conc sulphuric acid [H.sub.2 SO.sub.4 ]                                       de-ionised water to 1000 ml                                ______________________________________                                    

An alternative means of recording a hologram is to use thermoplasticfilm. Such film is commonly used in holographic interferometry. Its manyattractive features include rapid electronic processing and reusability.

The requirement of recording underwater with subsequent laboratoryreconstruction in air introduces additional factors to those mentionedabove upon which image fidelity may depend. Such factors include

(a) thermal gradients in the water,

(b) turbulence in the water,

(c) polarisation effects

(e) scattering

(f) absorption, and

(g) mismatch between the refractive index of the medium in which thehologram is recorded and that in which it is replayed.

Inevitably these processes result in an image which will have aresolving power below that of the equivalent situation in air. The imagewill also possess optical aberrations brought about by recording in onerefractive index medium and replaying in another.

A range of lasers were used in the experiments, as follows,

(a) Argon-ion

Manufactured by "Lexel" as Type 90-4. Contains oven stabilised etalonand delivers up to 1.5 W, continuous power at 514 nm, single frequencymode.

(b) Frequency doubled Nd-YAG

Manufactured by "Quantel". Delivers 250 mJ in a pulse of 15 ns durationat 532 nm, single-longitudinal mode (SLM). A second output of 50 mJ isalso available and serves as a reference beam.

(c) Ruby

Manufactured by "JK Lasers". Delivers up to 1 J in a 30 ns pulse at 694nm, SLM.

Agfa 8E56 plates were used with argon and YAG lasers and Agfa 8E75plates and film were used with the ruby laser. Chemical processing ofthe recorded holograms was carried out according to the proceduresoutlined above.

Whenever real image reconstruction was envisaged, the reference andreconstruction beams were collimated using an SORL Fourier lens. Thislens had a focal length of 300 mm, an f/5 aperture and a wavefrontaccuracy specified as better than λ/8 over its central 38 mm at 514 nm.Collimation of the beams were achieved to better than 2 mrad. Referencebeam angles of 30° to the normal with the holographic film were typical.This ensured that the spatial frequency of the system was well withinthe cut-off frequency of the film.

A series of holograms were recorded aimed at establishing the influenceof thermal gradients and turbulence in the water on the fidelity of thereconstructed image. Since thermal gradients and turbulence can both belinked to local changes in the refractive index of the water it is to beexpected that they will have local influences on the path length oflight travelling through the water. Such an effect, if severe enough,could appear in the hologram as localised interference fringes andobscure the inspection area. Although the "look around" properties givesome measure of compensation for this.

To simulate "worst-case" conditions, an immersion heater operating ataround 90° C. was placed in the observation tank in front of thetargets. A series of holograms at exposure times of 20 s down to 150 ms(the shortest obtainable with the above set-up and amplitude processing)showed that as expected the number of fringes recorded across the fieldof view decreased with exposure duration.

A photograph of one such hologram, taken from virtual imagereconstruction, is shown in FIG. 2 corresponding to an exposure durationof 150 ms. Spot temperatures in degrees Celsius at various points in thewater are also shown (numerals 18-21). In any field situation, though,the use of a pulsed laser with pulse durations of 10 to 50 ns would beessential because of the mechanical stability conditions required byholography. It would appear unlikely that, in these short time scales,thermal variations of the magnitude likely to be encountered offshorewould result in the appearance of secondary fringes in the hologram.These conclusions were verified by the short analysis which follows andby a series of similar holograms taken with both pulsed ruby andfrequency-doubled YAG lasers.

The mechanism by which interference fringes are formed is well known. Achange λ/2 in path length travelled by a given ray between twoconsecutive exposures will give rise to a dark fringe in the hologram.In this particular case the path length change is brought about by localtemperature variations causing a change in local density and hence inlocal refractive index of the water.

Assuming first order fringes, the refractive index change can be linkedto path differences by

    xΔn=λ/2                                       (3)

where x is the length of medium over which the path length changes, Δnis the refractive index change and λ is the wavelength of the laser.

Equation 3 can be linked to the temperature by assuming that, firstly,the spatial variation in refractive index is a step function and,secondly, that refractive index is a linear function of temperature overa suitably narrow range. Hence, we have

    n=aT+b                                                     (4)

where a and b are constants.

For small variations in n and T, we can say that

    Δn/ΔT=a                                        (5)

and hence,

    axΔT=λ/2                                      (6)

Equation (6) accordingly relates the appearance of fringes totemperature changes.

It has been shown that a temperature change of 20 degrees in pure waterat a nominal value of 20° C. will give rise to an approximately linearchange in n of 0.0008, yielding a value of 40×10⁻⁶ ° C.⁻¹ for theconstant a. This linear approximation is reasonable since a correlationcoefficient of 0.96 was obtained for published values of n and T overthis range. Hence, for a wavelength of 514 nm and assuming that therefractive index changes over a 2 cm path, we obtain

    ΔT=0.3° C.

Thus one dark fringe will appear for a temperature change of 0.3° C.over a 2 cm path of water. In contrast to the above the fringes whichappear in a single exposure hologram correspond to the change inrefractive index integrated over the exposure duration and may beregarded as an ensemble of double exposure holograms. Consequently thenumber of fringes over the field of view must decrease as the exposureduration decreases as previously concluded.

An order of magnitude calculation based on the previous analysis showsthat unrealistically large temperature variations would need to occurbefore such fringes would appear in the hologram.

A specially constructed target consisted of a water filled cell 20 asshown in FIG. 3 containing a small heating element 21 and threethermocouples 22,23,24 was used to verify the above analysis. Using theoptical arrangement shown in FIG. 4 comprising a lens 30, a diffusingscreen 31, a water cell 32, a collimated reference beam 33 and aholographic plate 34, a double exposure hologram was recorded with theheater off and on. A photograph taken from a real image reconstructionof the hologram is shown in FIG. 5. By examining the resulting hologramunder real image reconstruction the finite depth of the fringes could beseen. The measured temperatures indicate an actual thermal variation of0.25° C./fringe which is in reasonable agreement with the previousanalysis.

Like thermal gradients, the quality of underwater holograms may also beaffected by the presence of turbulent flow in the water. In suchsituations, turbulence may arise from, for example, surface waves orcurrents around structural members or, perhaps more severely, from theoperation of underwater inspection vehicles. Since turbulence may beinterpreted as localised fluctuations in the velocity and pressure ofthe water, such variations would be expected to cause minute changes inoptical path length resulting in the appearance of fringes in thehologram.

One simulation of the effects of small scale turbulence, utilising apaddle revolving at approximately 1 Hz, is shown in FIG. 6. A fewlocalised fringes are visible in the hologram. We can interpret theoccurrence of these fringes as arising from the small amounts of localheating which must inevitably be produced by the velocity and pressurefluctuations in the water.

A similar analysis to that conducted for thermal gradients againindicates that in the situation likely to be encountered, offshoreturbulence is unlikely to be a problem if short pulse lasers are used inhologram recording. Confirmation of this conclusion was obtained bytaking holograms similar to those outlined above using both pulsed rubyand frequency-doubled YAG lasers.

All our preliminary holograms showed that bright clear images could berecorded of objects submerged in clear tap water. Sea water, however,with its salinity, suspended matter and micro-organisms presents adifferent situation. It is to be expected that under these circumstancesscattering and absorption of light will have an influence on imagebrightness and apparent contrast of the subject. The extent of thesephenomena in a given medium dictates the limit of visibility.

Scattering of light in an attenuating medium occurs when light interactseither with particles suspended in the medium, or, with inhomogeneitiesin the medium and consequently is deviated from its original path. Insea-water, transparent micro-organisms and suspended particles are muchlarger than the wavelength of light. Thus, unlike atmosphericscattering, scattering in sea-water is relatively constant over thevisible spectrum. Absorption, on the other hand, is a thermodynamicprocess which results in the loss of irradiance of a beam of light as ittraverses the medium and, as such, is strongly wavelength dependent.Other mechanisms such as fluorescence and photosynthetic absorption aresmall enough to be negligible.

When a collimated beam of light, with an initial irradiance E_(o).passes through an attenuating medium, its irradiance E_(x) at a distancex from the source is given by

    E.sub.x =E.sub.o exp(-αx)                            (7)

where α is the total attenuation coefficient. In water this coefficientis primarily the sum of attenuation due to scattering and that due toabsorption. Typically, α is of the order of 0.04 m⁻¹, 0.20 m⁻¹ and 0.34m⁻¹ for distilled waters coastal water and bay water respectively, allmeasured at 510 nm.

A more convenient way of expressing the attenuation of light is in termsof the "attenuation length" (α⁻¹) of the medium. The attenuation lengthis defined simply as the reciprocal of the attenuation coefficient andis expressed in "meters". Expressing the above figures in terms ofattenuation length we have about 25 m for distilled water, 5 m forcoastal water and about 3 m for bay water. It is generally agreed thatthe visibility limit of a dark object in water, near the surface indaylight is around four attenuation lengths.

The variation in attenuation length with wavelength for distilled wateris shown in FIG. 7. The peak transmission occurs at around 480 nm. Insea-water, with its dissolved yellow substances caused by breakdown ofanimal and plant matter, we would expect this "transmission window" toshift towards the green region of the spectrum. At this peak about 60%of attenuation is due to scattering by particulate matter, whereas therest is due to absorption.

The above behaviour indicates that for long-range underwater work,lasers such as argon-ion or frequency-doubled Nd-YAG, which havewavelengths in the blue-green region of the visible spectrum, will bethe most suitable choice.

An effect of scattering common to all underwater optical imaging systemsis the reduction in contrast and apparent brightness of the image. Twomechanisms are responsible for this: firstly, light from the irradiatingbeam can be backscattered towards the film creating a "luminous fog"through which the target is viewed; secondly, light reflected from thebrighter parts of the target is forward scattered into the line of sightof darker parts of the target. Both mechanisms cause the darker parts ofthe object to appear brighter than they really are relative to thebright areas resulting in a decrease in contrast of the image. This lossof contrast ultimately degrades resolution.

In holography, image contrast is further dependent on fringe visibilityin the recorded hologram, which is, in turn, dependent upon the relativeplanes of polarisation of the interfering beams. When recording ahologram, only object light which is polarised in the same plane ofvibration as the reference light can actually interfere to produce therequired hologram. Conventionally the object and reference beams areboth polarised in the vertical plane of the electric vector. However,light scattered back towards the film plane from small particulates inthe water may suffer some depolarisation resulting in some light of thewrong polarisation reaching the film. Although this light cannotcontribute to the recording of the hologram it does raise the overallbackground level of the film thereby reducing the signal-to-noise ratio.

Preliminary investigation of the holograms taken earlier did indeedindicate a loss of brightness for holograms taken in sea-water overthose taken in air. Measurements of the polarisation of the object beamirradiating the film showed it to contain a component of horizontallypolarised light at around half the irradiance of the original verticalpolarisation.

The magnitude of the depolarisation was studied using the arrangementshown in FIG. 8 consisting of a collimated laser beam 40 passing througha water tank 41, together with a moveable detector and prism 42. Theirradiance and state of polarisation of light scattered from acollimated laser beam as it traversed a tank of sea-water was measuredat various angles around the beam direction. The state of polarisationis expressed as the ratio of the irradiance of the vertical component,E_(V), to the sum of both horizontal and vertical components, E_(V)+E_(H), as

    p=E.sub.V /(E.sub.V +E.sub.H)                              (8)

A value of p close to unity indicates a beam which is stronglyvertically polarised, whereas, p tending towards zero indicates a strongdegree of horizontal polarisation.

A graph of p and total irradiance, E_(H) +E_(V), against scatteringangle is shown in FIGS. 9(a) and 9(b) resp. The polarisation state ofthe straight through beam (0°) remained unchanged from its initial valueof p very nearly equal to one indicating that the beam was not sufferingany significant depolarisation. At angles greater than 10° theproportion of horizontally polarised light increased peaking at ascattering angle of about 90°, corresponding to a p-value of 0.62. Lightbackscattered at angles greater than 90° contains as much as 30% of thehorizontally polarised component, indicating that such light reachingthe film would contribute to the overall background level. The overallirradiance of the light decreases as the scattering angle increases. Thehigh intensity of scattered light which is visible at these smallforward angles is believed to be due to transparent plankton having arefractive index close to that of water and small point-to-pointvariations in refractive index caused by thermal effects and salinitygradients as discussed earlier. Wavefronts passing through sea-waterwill, therefore, be distorted and the resolving power of any viewingsystem will be expected to deteriorate over its comparable performancein air. Small angle scatter is not expected to have much effect on thepolarisation of light. Scattering from larger particles, however, willhave some depolarising effect and this would account for the lowervalues of P obtained at larger angles.

Another likely cause of depolarisation is that due to the lightreflected from the target itself. In a series of experiments aimed atestablishing the magnitude of this effect, parallel laser light wasreflected from rough surfaces 46 such as corroded aluminium and groundglass and the light scattered at various angles monitored using thearrangement shown in FIG. 10 comprising a collimated laser beam 44 andpolarisation detector 45. A graph of p and total irradiance, E_(H)+E_(V), against scattering angle is shown in FIG. 11. As expected,significant depolarisation, up to 20%, of the light is experienced atall scattering angles. The extent of depolarisation, however, decreasesas the scattering angle increases, with p eventually reaching a maximumwhen the scattering angle equals that of the angle of incidence, that isat 45° to the normal. The total irradiance is also a maximum at thisangle. Hence, at that angle the beam suffers the least amount ofdepolarisation.

The above effects can be minimised if a vertically orientated linearpolariser is placed in front of the film plane so that only lightpolarised in the required plane reaches the film. Hence the backgroundirradiance will be reduced. Furthermore when exposure values areestimated a similar filter placed across the exposure meter will ensurethat only the correctly polarised light will be measured.

Advantageously, the concepts of circular polarisation can be exploitedto improve image contrast. Light which is circularly polarised in aparticular direction changes its "handedness" each time it is reflected.For example, light which is originally "left-hand" polarised will become"right-hand" polarised after one reflection and will return to"left-hand" polarised after two reflections. Generally, lightbackscattered from particulates in sea water will be reflected once,whereas, light reflected from rough objects will experience more thanone reflection. If left-hand (say) circularly polarised light is used inobject illumination, scattered light will be predominantly right-handpolarised and light reflected from the object scene will possess anapproximately even mix of left- and right-hand components. Placing aright-hand polariser at the film plane will ensure that only lightreflected from the object will reach the film. Thus the contrast of theimage will be improved by removing unwanted scattered light. Thisprocess, though, is obviously wasteful of energy since about half thelight reflected from the object is thrown away. The process, though,will only be effective with rough objects, specular reflectors willexperience only one reflection and hence will suffer a reduction incontrast. A further quarter-wave plate is needed behind the circularpolariser to linearise the light.

Additional techniques for the reduction of back-scatter which have beenfound advantageous in underwater photography include "volume reduction"and "range gating". In the former, the scattering volume which is commonto both source and receiver fields is reduced by increasing theseparation between source and receiver. For a holographic camera thiswould be accomplished by increasing the distance between the emittedobject beam and the film plane. In practical terms this solution mightput unreasonable constraints on system geometry. Range gating is onesolution to the problem of backscatter. The receiver is electronicallygated in conjunction with the use of a pulsed laser as the illuminatingsource. The photosensitive medium is exposed to light only at theinstant the pulse reflected from the target reaches it and then switchedoff. In this way, any light backscattered towards the film byparticulates in the water will not be seen by the receiver. Analternative concept which can be exploited in holography is known as"coherence gating". In this technique, the reference beam and objectbeam paths are matched to within the coherence length of the laser. Thecoherence length is adjusted to correspond to the distance betweenholographic film and subject. Hence only light reflected from the objectwill meet the conditions for interference, light backscattered from thewater will not meet the conditions and will, therefore, not be recordedon the hologram.

To compare and contrast the resolving power of holograms recorded ofunderwater objects with the equivalent holograms taken in air theoptical arrangement as previously shown in FIG. 1 was used. Hologramswere recorded, both, with and without the observation tank in place. Thetarget for all resolution measurements was a standard resolution chartpossessing a series of vertical and horizontal bars in the range 1 to2281 p/mm. The resolution of the reconstructed real image was measuredusing a travelling microscope fitted with a 10x Ramsden eyepiece with anoverall system magnification of 20x. All holograms were recorded on Agfaholographic plates type 8E56HD and processed using a pyrogallol baseddeveloper (Agfa formulation GP 62) and bleached in a para-benzoquinone(PBQ) based bleach (Agfa formulation GP 432).

Holograms were recorded with the unexposed plate in the plate holder andthe resolution chart in position A of the optical arrangement. Afterprocessing according to the method outlined above the hologram wasrotated through 180° and illuminated through the back of the plate. Thecorresponding real image is reconstructed in position A'. Collimation ofreference and reconstruction beams was accomplished using a lens with afocal length of 300 mm and aperture of f/5. The wavefront accuracy isλ/8 over the central 38 mm of its aperture at 514 nm. In order toilluminate as large an area of the holographic plate as possible theentire lens aperture was used to expose a roughly elliptical area of 65mm×75 mm of the film. The lens was collimated to an estimated divergenceof no more than 2 mrad. A reference beam to optic axis angle of 30° keptthe spatial frequency of the system below that the cut-off frequency ofthe film.

The laser used in the experiments was an argon-ion (Lexel type 90-4)delivering up to 1.5 W single frequency at 514 nm. The entire set-up wasmounted on a vibration isolated table.

A number of holograms were recorded in order to monitor the opticalresolution achievable when recording the holograms underwater.Initially, two holograms were taken under the conditions outlined abovebut with no observation tank in position. In other words the hologramswere recorded entirely in air in order to establish a reference point.These holograms were taken at target-to-film distances of 550 and 1000mm respectively. A second pair of holograms were the recorded at thesame target-to-film distances but with a perspex observation tank inplace. In this case the perspex wall was nominally 10 mm thick and thedistance between the front wall of the tank and hologram plane was 240mm. A third pair of holograms were recorded as above but with turbidwater in the observation tank. In this situation the holograms wererecorded at in-water paths of 300 and 750 mm respectively. In all casesthe film, perspex interface and target were parallel to each other andon the same optic axis.

The experimentally obtained resolving powers are shown in Table 4. As areference point a resolution of 57 lp/mm was measured directly on theoriginal resolution chart using the measuring microscope describedearlier when illuminated by reflected laser light at 514 nm.

                  TABLE 4                                                         ______________________________________                                        Resolving Power Measured from Underwater Holograms                            Location   Total film Path dist of                                                                              Measured                                    of target  to target dist                                                                           target in water                                                                           Resolution                                  ______________________________________                                        In air      550 mm                22 lp/mm                                    In air/in tank.sup.1                                                                      550 mm                20 lp/mm                                    In water/in tank.sup.1                                                                    550 mm    300 mm      18 lp/mm                                    In air     1000 mm                 9 lp/mm                                    In air/in tank.sup.1                                                                     1000 mm                 8 lp/mm                                    In water/in tank.sup.1                                                                   1000 mm    750 mm       7 lp/mm                                    ______________________________________                                         Note 1: This distance includes a film plane to tank separation of 240 mm      and a nominal tank wall thickness of 10 mm.                              

The figures obtained for resolving power of underwater holograms show adecrease over the reference holograms recorded in air as, firstly, aperspex interface is placed in the optical path and then, secondly, awater interface is added to the path. The ultimate reduction inresolving power is from 22 to 18 lp/mm. These figures should becontrasted with those obtained by underwater photogrammetry whichindicate a resolving power of around 0.5 lp/mm for similar viewingconditions in sea water.

In the field, measurement of resolving power is not a very meaningfulfigure considering that the object may be rough, poorly reflecting andlow contrast. Being able to measure a particular set of bars on aresolution target does not really help in determining whether or not aparticular surface feature can be visualised using holography. Toillustrate this a hologram was taken of an engineering test piece: apolished titanium block with a stress-induced crack in it. The hologramwas taken under the conditions outlined above with a totalobject-to-film distance of 550 mm and a distance in water of 300 mm. Aphotograph taken from the reconstructed real image is shown in FIG. 12.The crack, which was measured to be 40 μm across the root, is clearlyvisible. A reconstruction from a second hologram, taken under identicalconditions to those above, of a corroded weld specimen is shown in FIG.13.

A further range of holograms were taken with the objects submerged in"live" sea water. The total attenuation length, α⁻¹, of the water wasmeasured, using a simple arrangement of collimated laser beam traversingthe water filled tank, as 0.56 m at 514 nm. This attenuation correspondsto a light loss of some 80% over a 1 m beam path. The path was through a400 mm length of sea water. In this unoptimised system, a real imageresolution of 5 lp/mm was determined for the central on-axis parts ofthe object.

When a hologram is recorded underwater and replayed in air, thereconstructed image will suffer from optical aberrations due to thedifference in refractive index between the two media. For points on-axisthe image will replay closer to the film plane in the simple ratio ofthe refractive indices. This latter fact will be modified by thepresence of the perspex interface.

Measurements of the image shift were made using the optical arrangementshown earlier, but this time provision was made to move the targetposition laterally with respect to the optic axis. For each hologram aground glass screen was used to view the real image and thereconstructed image position determined. Table 5 shows the measuredimage shifts for a number of target locations.

                  TABLE 5                                                         ______________________________________                                        Image Shift Measured from Underwater Holograms                                Object Position                                                                           Image Position Image Shift                                        On-axis                                                                              Off-axis On-axis  Off-axis                                                                              On-axis                                                                              Off-axis                              ______________________________________                                        417 mm  0 mm    309 mm    0 mm   108 mm  0 mm                                 417    230      288      227     129    13                                    417    435      149      388     268    47.sup.1                                              194      432     223     3.sup.1                              ______________________________________                                         Note 1: In this case two image positions were obtained, the first one         corresponding to the vertical bars on the resolution target and the secon     one corresponding to the horizontal bars of the target.                  

The data shows that for on axis points the image shift is in accordancewith the shift predicted from a simple refractive index ratio. As thetarget, however, is moved laterally with respect to the optic axis themeasured image shift increases beyond that expected by simple theory.The reasons for this being that now the light rays travelling from thehologram to the image position are traversing paths which aresubstantially different from those encountered in recording. Inparticular, the refraction of light encountered at the air/perspex/waterinterface during recording does not occur in replay. The replayed raysdo not converge to the same point from which they emanated in recording.As a consequence of this, two distinct image points are formed onreconstruction: one for the horizontal resolution bars and one for thevertical resolution bars. In other words the image is astigmatic. As theobject is moved further from the optic axis the difference between thetwo image positions increases.

Analysis of underwater holograms has shown that when a real image isreconstructed in a medium of lower refractive index (air) from ahologram originally recorded in a medium of higher refractive index(water) the resulting image will suffer from optical aberrations. Theorigins of this may be understood by reference to FIG. 14 which showsthe path of two rays emanating from an object point, P, located on theoptical axis with respect to the hologram centre, O. The refractionsuffered by these two rays at the media boundaries serves to produce avirtual image of P located at point, P'. Hence on reconstruction of thereal image, the image of point P will be located at point P' a distanceOP' in front of the hologram. The actual position of P' cannot beprecisely located by simple tracing of the paths of two rays as shown,since, any other two rays which subtend a different angle will locate P'at a different position along the optic axis. This situation isanalogous to spherical aberration produced by a lens. In practice, theposition of P, will be determined by identifying the circle of leastconfusion located at the waist of the ray bundle formed by tracing thepath of all rays through the three media.

Using a paraxial approximation, the expected image shift for on-axispoint objects has been calculated and confirmed to a limited extent byexperiment as discussed above.

For points off-axis, the situation is complicated by the appearance ofastigmatism in the image as discussed in the following section.

Experiments involving the reconstruction of real images from hologramsof off-axis point objects in water have revealed a considerable degreeof astigmatism. The origins of this astigmatism would appear to lie inthe fact that the hologram is being recorded in one refractive indexmedium and being replayed in another.

FIG. 15 shows the basic geometry adopted in recording the hologram. Theorigin O of the co-ordinate system is taken as the centre of thehologram H. The object point P is located off-axis in the xoz plane atan arbitrary distance z_(w) in water. Shown in the figure is therefraction of the meridional rays, PA and PB, the sagittal rays, PC andPD, and the principal ray, PO, at the water/glass and glass/airinterfaces. For an object point off-axis in only one plane, the twosagittal rays will be symmetrically orientated with respect to theprincipal ray, thereby subtending equal angles with the refractingsurfaces and, hence, with the hologram plane. Whereas, the optical pathlengths of the sagittal rays are equal in each of the three media thiswill not be the case for the tangential rays, PA and PB. In this case,the angles subtended by the rays at the hologram plane OAA' and OBB' areunequal. This situation is identical to that pertaining to the originsof astigmatism in a lens system.

In the context of holography, the aberrations discussed will be presentin the reconstructed real image. The source of these aberrations is,however, not connected with the holographic recording process. In orderto observe the astigmatic effects outlined above it is only necessary toview, from a position in air, and object immersed in water. Theholographic process faithfully records the astigmatic image which can beseen by any observer.

In the absence of significant monochromatic aberrations a point imagewould be observed at an equivalent position in the negative x and zposition. Upon illumination of the hologram with the reference beam theoriginal wavefronts emanating from the point object are reconstructed,maintaining their original orientation with respect to the holographicplate. Thus the meridional rays AA' and BB' and sagittal rays CC' andDD' proceed outwards from the hologram plane in the (-x,-z) directionand, failing to meet any refracting surfaces, form a point image attheir intersection. However, the previous analysis of the recordingstage shows that it is highly unlikely that the meridional and sagittalrays will come together at a common focal point. The reconstructed imagemay be expected to exhibit some degree of astigmatism. As might beexpected from geometrical considerations this astigmatism has been foundto disappear for axial object points leaving, in this case, onlyspherical aberration to consider.

The variation in path lengths from the object point via the refractingsurfaces to the hologram plane is analogous to the zone concepts of lensimaging. The astigmatic nature of off-axis point images is, therefore,not unexpected.

In considering the aberrations associated with viewing a point objectlocated in water it is important to gain an impression of how the degreeof aberration varies with experimental parameters. This analysis isoutlined in the following section with particular reference to thedetermination of the location and difference between the astigmaticimages.

FIG. 16 shows the refraction of a ray at water/glass and glass/airboundaries. A ray originating from a point object P located in the watersuffers refraction at both water/glass and glass/air interfaces andpasses through a point R located in air. The angle subtended by therefracted ray to the normal as it leaves the point P in water is denotedby θ_(w), θ_(g) is the angle subtended by the ray to the normal at thewater/glass interface and θ_(a), is the angle subtended by the ray tothe normal at the glass/air boundary. The co-ordinate distances alongthe optic axis in the respective media are denoted by z_(a), z_(g) andz_(w) for air, glass and water. Tracing the ray back through the mediaas if refraction were absent it appears to meet the optic axis at apoint S.

An equation defining this ray may be given as a function of the angleθ_(a), as

    θ.sub.a =sin.sup.-1 [(n.sub.w /n.sub.a) sin θ.sub.w ](9)

More usefully the ray path may be defined in terms of the location ofthe point R above the optic axis. Hence,

    y.sub.a =z.sub.a tan θ.sub.a +z.sub.g μ.sub.ag sin θ.sub.a [1-(μ.sub.ag sin θ.sup.2)].sup.-1/2 +zwμ.sub.aw sin θ.sub.a [1-(μ.sub.aw sin θ.sub.a).sup.2 ].sup.-1/2(10)

where μ_(ag) =n_(a) /n_(g) and μ_(aw) =n_(a) /n_(w)

Equation 10 may be solved by iteration and a value determined for θ_(a)relating to a given point R.

The ray emanating from P is just one of a family of such rays, whichdepend on the observer's viewpoint, each of which intersects the opticaxis at some point S. The position of S will get progressively closer toP as the divergence of the ray PS decreases. Consider a pair of raysequidistant from, but infinitesimally close to R in the meridional (yz)plane of FIG. 14. These two rays will intersect at a point T at somedistance yc above the optic axis. Adjacent pairs of rays will map outthe loci of all virtual image positions and describe a caustic curve asshown in FIG. 17. Hence it can be seen that two image positions areobtained for each ray. one on the optic axis (the sagittal image) andone on the caustic curve (the meridional or tangential image). Theequation defining the caustic curve, thus contains all the informationrequired to locate the astigmatic images associated with viewing thepoint P from any given location in air. It should be realised that ifthe position of best focus is being looked for in reconstruction of thehologram, the observer will select the point where the ray bundleconverges to its minimum diameter. This is the circle of least confusionand it will occur somewhere between the two astigmatic image positions.

In three dimensions, the loci of virtual image positions form a surfacewhich is obtained by rotating the curve of FIG. 17 about the z-axis. Theshape of this virtual caustic surface is a result of the increasingdivergence of the refracted rays with increasing distance from thez-axis. By virtue of this rotational symmetry the projected rays can beseen to form a series of cones whose apex lie at increasing distancesalong the z-axis with decreasing radial distance of the projected raysfrom the z-axis measured in the plane of the media interface. The locusof intersection of any two adjacent cones (a circle) represents across-section of the caustic surface in a plane perpendicular to thez-axis. The intersections of an infinite series of such cones can beconsidered to generate the entire caustic surface.

Since the ray RS is actually a tangent to the caustic surface as shownin FIG. 16, it is possible to express the co-ordinates of any point (yc'zc) of the point T on the caustic surface as a function of θa, asfollows,

    z.sub.c =Z.sub.g μ.sub.ag cos.sup.3 θ.sub.a [1-(μ.sub.ag sin θ.sub.a).sup.2 ].sup.-3/2 +z.sub.w μ.sub.aw cos.sup.3 θ.sub.a [1-(μ.sub.aw sin θ.sub.a).sup.2 ].sup.-3/2(11)

    y.sub.c=z.sub.g μ.sub.ag (1-μ.sub.ag.sup.2) sin.sup.3 θ.sub.a [1-(μ.sub.ag sin θ.sub.a).sup.2 ].sup.-3/2 +z.sub.w μ.sub.aw (1-μ.sub.aw.sup.2) sin.sup.3 θ.sub.a [1-(μ.sub.aw sin θ.sub.a).sup.2 ].sup.-3/2                           (12)

from a given observation point R in air, Equations 10, 11 and 12 permita determination of the astigmatic image points T and S and hence thedifference in location between the two images. The extent of thesagittal and meridional line images may also be determined.

In any practical arrangement where a point object P in water is viewedfrom an observation point in air the viewing aperture, in our case thehologram, will have finite dimensions. The astigmatic images consist oftwo separate line segments possessing finite length and width. Thesedimensions are determined by those rays of the ray bundle associatedwith the extended aperture in the meridional and sagittal focal planes.If the previous analysis is applied to a point at the centre of theaperture the location and separation of the line images may bedetermined since the image points T and S locate the centres of bothline images. Since often an evaluation of the astigmatic difference isof primary concern, the point image analysis detailed above provessufficient as illustrated in FIG. 18. This information can be used inassessing the extent of the aberrations associated with imaging a finiteobject through glass and water.

Table 6 shows some data calculated for a hologram of 70 mm dimension.

                  TABLE 6                                                         ______________________________________                                        Analysis of Sagittal and Meridional Images                                    for Underwater Holograms                                                      (All dimensions are in millimeters)                                           ______________________________________                                        Off-axis distance (x.sub.R)                                                                     500         100                                             Object point co-ordinates (x, y, z)                                                              0, 0, 737  0, 0, 737                                       Meridional image point (x, y, z)                                                                57.1, 0, 521.2                                                                            0.6, 0, 624.1                                   Length of meridional line                                                                        0, 0, 588.4                                                                               0, 0, 627.8                                    Sagittal image point                                                                            8.5         0.4                                             Length of sagittal line                                                                         10.6        2.5                                             Astigmatic difference                                                                           88.1        3.8                                             ______________________________________                                    

Where it is required to evaluate the dimensions of the line images orsimply to visualise the convergence of rays from the aperture to themeridional and sagittal focal planes then Equation 1 provides the basisfor a "spot diagram" analysis. The spot diagram serves as anillustration of the cross-section of the ray bundle by an array ofpoints at various distances from the aperture. This technique serves asa useful method of visualising the image forming process.

On the basis of the foregoing analysis, we have devised cameras forlong- and short-range under-water holography. By way of example, along-range camera comprises an enclosure 50, having a port 51 forservices and a window W for observation of an object (not shown).Windows 52,53 are provided for the recording beam 54a,b. The radiationsource comprises an amplified laser 55 with beam splitters andassociated half-wave plates 56,57 to derive the reference and recordingbeams. Prisms P1-P6 are provided to fold the various beams and constrainthem within the available space. A collimating lens 58 and mirror 59direct the reference beam through a filter assembly 60 on to a film 61in a carrier 62. Circular polarisers 63,64 are provided in the path ofthe recording beams.

In the short-range camera shown in FIG. 19 (b), only one recording beamoutlet is provided.

To produce successful holograms underwater requires careful selection ofa laser with the required holographic performance and configuration forsubsea use. Our experience indicates that two generic classes of laserare most appropriate for use in the specific holographic cameraenvisaged here, namely, the ruby laser or the frequency-doubled Nd-YAGlaser. Of these two classes the ruby laser has its output in the redregion of the optical spectrum (λ=694 nm) and is most commonly used inindustrial holography. The frequency-doubled Nd-YAG laser, though rarelyused in holography up until now, has an advantage over ruby forunderwater use because its output wavelength is in the green region ofthe spectrum (λ=532 nm) and closely matches the peak transmission windowof sea water. In normal circumstances for underwater holography of largevolume subjects from long stand-off distances the Nd-YAG would be theideal choice because of its wavelength advantage. For holography of asmall subject at a short stand off distance, however, the wavelengthadvantage is not so significant and successful holograms could be madewith a ruby laser. For that reason, holographic cameras could beenvisaged using either Nd-YAG or ruby.

A significant advantage of current Nd-YAG laser over the ruby is interms of its pulse repetition rate. Whereas a maximum repetition rate of5 pulses a minute of holographic quality can be attained for ruby, YAGsystems can be operated at around 1 Hz. Ruby lasers could perhaps bedesigned to operate at faster repetition rates but at a cost and sizepenalty.

The mechanism by which energy is pumped into the laser medium also hassome bearing on the size and performance of the laser. Traditionally,solid state lasers are pumped by a capacitively discharged flashlamplying parallel to the crystal. Depending on the performance required bythe laser this generally implies the use of a physically large powersupply. Recent progress in solid state laser technology has led to thedevelopment of the diode-pumped Nd-YAG laser. Such lasers appear tooffer a more efficient and compact means of coupling energy into thelaser medium.

The output energy required for successful underwater holography dependsto a great extent on both the condition of the water and that of theobject. We have found that for short-range holography an energy of 50 mJwould be sufficient, whereas, for long-range work an energy in excess of250 mJ is needed. The 50 mJ of energy can be obtained using a laser witha single crystal rod (the oscillator). For higher energy, a secondcrystal is needed to amplify the oscillator energy.

With these points in mind a specific laser has been identified as beingparticularly attractive from the point of view of performance and size.The laser under consideration is a pulsed system using, at present, rubyas its active medium. In most aspects of its performance, for example,linewidth, pulse duration and output energy, it is at least as good asthe competitive lasers from other manufacturers. In some aspects, asindicated below, it is vastly superior to any other system currentlyavailable:

(a) Of prime consideration is the fact that this particular laser hasbeen designed as a holographic system from the outset rather than as an"improved" industrial laser. Hence its performance is optimised in thecrucial areas of holographic performance and, consequently, could beeasily configured to holographic camera specification.

(b) A second important area where this particular laser scores over thecompetition is in its overall size. The laser head, at around 750 mm 3375 mm across depending on ultimate configuration, is approximately 1% ofthe volume of the equivalent commercial system. Its power supply atabout 600 mm×600 mm×100 mm, is barely 2% of the volume of the commercialequivalent. Obviously these size advantages are extremely relevant toany system which needs to be configured for subsea use.

(c) A third significant consideration for subsea use, which this laseraddresses, is that the laser should maintain its holographic performanceover a wide range of ambient temperatures. Hence, the laser should beable to produce high quality holograms with a variation in cooling watertemperature over several degrees. This particular system claims to beable to produce good holograms over a temperature variation of thecoolant of ±7.5° C. This performance should be compared with that of atypical system which will only guarantee good quality holograms over a±0.5° C. spread.

The above laser can be configured in either oscillator-only mode oramplified mode and can be adapted for ruby or frequency-doubled YAG.

An alternative method of camera construction, to mounting the entirelaser and optical components in the camera head, is to mount the laserhead on a ship or platform and carry the light to the object via anoptical fibre. The most suitable arrangement is one in which both objectand reference beams are carried by an optical fibre. The reference fibrecarries light directly to the film holder, whereas the object fibrecarries light to the scene of interest. In this arrangement only thefilmholder and fibres need be taken underwater. To maintain thecoherence of the laser light single mode fibre should be utilised.Because such fibre has a core diameter of the order of only 5 μm,coupling of light into the fibre is difficult and when taken togetherwith the high attenuation experienced at visible wavelengths, light lossis high. However, successful holograms have been taken using 2 m lengthsof fibre. The source used was an argon-ion laser. To ensure that theplanes of polarisation of the exit beams were parallel, polarisationrotators have to be included in the optical path. Preferably, a pulsedlaser is used as the radiation source. Care must be taken because thehigh radiance of such lasers can cause melting of the input end of thefibre.

The choice of film size has a considerable bearing on image resolution:the larger the diameter of the film the better is the resolution. For 70mm film the theoretical image resolution is around 8 μm for ruby laserlight at a target distance of 1 m. For 35 mm film the theoreticalresolution is about 16 μm. It should also be noted that the viewingangle will be less for the smaller film size.

The quality of the recorded hologram depends to a large extent on thechoice of film type and processing techniques. Among the factors to beconsidered are exposure sensitivity, contrast, resolution andsusceptibility to emulsion shrinkage.

An alternative means of recording a hologram is to use thermoplasticfilm. Such film is commonly used in holographic interferometry. Its manyattractive features include rapid electronic processing and reusability.

Ideally the film holder should be able to accommodate holographic filmin lengths corresponding to around 250 exposures. This latter numberwould allow for most applications of a "survey" nature. The film holdershould be motor driven at up to 1 fps. The size of film chosen obviouslyhas a bearing on size and performance of film holder and eventuallyholocamera size.

For high resolution holography it is essential that the film be held asflat as possible between thin flat glass plates and should not bestretched or put under any strain during exposure. This requirementdictates that care should be taken in choosing the film transportmechanism.

Generally, high resolution reconstruction of images from holographicrecordings requires that any optical aberrations in the system bereduced to a minimum. For systems employing monochromatic light theaberrations of concern are spherical, coma, astigmatism, distortion andfield curvature. Assuming recording and replay in air. the aboveaberrations can be reduced to zero, for point objects, only if aparallel reference beam is used. at both recording and replay stages,and also, if the recording and replay wavelengths are identical.

For the more realistic case of an extended object the situation issimilar to that mentioned above: although it is now not possible toreduce the aberrations to zero, they can be minimised if theaforementioned conditions are met.

The collimating lens is chosen to be of a sufficient diameter toentirely illuminate the holographic film. Since it is also not desirableto utilise the maximum diameter of the lens because of the introductionof edge effects the chosen diameter of lens has a firm bearing on theultimate camera dimensions. To adequately illuminate 70 mm film, forexample, a lens diameter of 150 mm is desirable. Consequently thisdetermines a focal length of around 300 mm. For 35 mm film therespective sizes are 50 mm and 100 mm respectively.

A possible method of minimising camera volume would be to carry thereference beam through a length of optical fibre. This can beaccomplished successfully at the lower irradiance levels needed for thereference beam.

For optimum holographic recording in terms of brightness and contrast itis generally necessary to ensure that the paths travelled by the objectillumination beam and that of the film illumination beam (the referencebeam) are identical. This condition can, of course, only be fulfilledfor one specific object plane. In practice, though, provided that theentire scene of interest and the reference beam path are matched towithin the coherence length of the system good, bright holograms will beobtained. For the lasers under consideration here coherence lengths ofthe order of a meter or more are typical.

For optimal matching, path length compensation may be incorporated intothe camera. Preferably, this would be preset, prior to deploying thecamera, for a particular target range. On the other hand, it may bethought desirable, for reasons of stability, to have the reference beampath fixed at the most appropriate length for a majority of situations.

In recording a hologram only light beams polarised in the same plane ofvibration can actually interfere to produce the required hologram.Conventionally the object and reference beams are both polarised in thevertical plane of the electric vector. However, light scattered backtowards the film plane from small particulates in the water may suffersome depolarisation resulting in some light of the wrong polarisationreaching the film. Although this light would not contribute to therecording of the hologram it could raise the overall fog level of thefilm and is best removed. A similar effect occurs with light reflectedfrom specular objects in the observation scene itself. These effects canbe minimised if a vertically orientated linear polariser is placed infront of the film plane so that only light polarised in the requiredplane reaches the film.

For some objects the concepts of circular polarisation can be exploited.Light which is circularly polarised in a particular direction changesits "handedness" each time it is reflected. For example, light which isoriginally "left-hand" polarised will become "right-hand" polarisedafter one reflection and will return to "left-hand" polarised after tworeflections. Generally, light scattered from particulates in sea waterwill be reflected once, whereas, light reflected from rough objects willexperience more than one reflection. If left-hand (say) circularlypolarised light is used in object illumination, scattered light will bepredominantly right-hand polarised and light reflected from the objectscene will possess an approximately even mix of left- and right-handcomponents. Placing a right-hand polariser at the film plane will ensurethat only light reflected from the object will reach the film. Thus thecontrast of the image will be improved by removing unwanted scatteredlight. This process, though, is obviously wasteful of energy since abouthalf the light reflected from the object is thrown away.

The process will only work with rough objects, specular reflectors willexperience only one reflection and hence will suffer a reduction incontrast. A quarter-wave plate is needed behind the circular polariserto linearise the light.

To reduce wavefront distortion to a minimum all ancillary componentssuch as mirrors, prisms and beam splitters should have their criticalsurfaces flat to within λ/20. Additionally, all transmissive surfacesshould be anti-reflection coated to minimise light loss.

The beam splitter as its name suggests divides the intensity of the beaminto two parts: one part forming the reference beam and the otherforming the object beam. The split is not equal. Most light is neededfor object illumination, since much of it is lost in scattering andlarge angle reflection, while only a small portion need form thereference beam. The portion forming the reference beam is easier todetermine since it is this beam which governs overall exposure of thefilm.

The beam splitter directs about 2% of the incident beam into thereference path. A half-wave plate at the output rotates the plane ofpolarisation of the reference beam through 90° so that it is in the sameplane as the object beam. A similar half-wave plate at the input to thebeam splitter controls the relative intensity of both beams such thattheir ratio can be varied.

Planoconvex lenses are chosen for all applications where a focused laserbeam may cause air breakdown. The curved portion of the lens is placedon the opposite side from the incident beam to ensure that lightreflected back down the system cannot be refocused in air.

In conventional photography, to prevent unwanted light reaching the filmplane a shutter would normally be placed in front of the film and openedat the required time. This could also be done for holography. However,because of the general insensitivity of holographic film to light andthe monochromaticity of laser light it is only necessary to position anarrow band wavelength selective filter over the film such that onlylight from the laser can reach the film and expose it. It should bepossible to fabricate the interference as a composite unit together withcircular and linear polarisers.

An underwater housing is necessary to protect the camera from ingress ofwater and external pressure effects. The housing should be designed towithstand a pressure of 30 bar. An optical window should be incorporatedto enable emittance and return of light. It is envisaged that thecomplete system be mounted on a remotely operated vehicle (ROV). Powerwill be drawn from the ROV system.

The performance and parameters of the holographic replay system have acrucial bearing on the fidelity of the reconstructed image. Ideally, theoptical system used in replay should be matched to that of the camera inboth geometry and wavelength. Thus the reconstruction system must bedesigned in conjunction with the camera to ensure that any compromisesmade in one do not adversely affect the performance of the other.

Some of the elements required in the reconstruction system share commonperformance specifications with the similar component in the recordingsystem such as those relating to ancillary mirrors, collimating lens andfilm holder.

The specific features of replay worthy of particular mention are thoserelating to the quality of the reconstuction beam and the minimisationof optical aberrations.

We have found that the use of a collimated reference beam at bothrecording and replay stages was desirable for the attainment of highfidelity images. It is necessary that a high degree of collimation ismaintained for both beams with 1 mrad being an acceptable upper limit.The diameter of the collimating lens should at least match that of therecording collimator.

We have also found that for high image fidelity recording and replaywavelengths should preferably be identical. The pulsed lasers used inrecording the hologram are unsuitable for reconstructing an image uponwhich high resolution measurement has to be performed. Hence, it isnecessary to pump a tuneable dye laser with a suitable continuous lasersuch as an argon-ion. Suitable dyes are available to allowreconstruction at both ruby and frequency-doubled wavelengths.

The situation here is of course complicated by the fact that theholograms are recorded in water and subsequently replayed in air.Aberrations will be introduced into the system, the most severe of thesebeing astigmatism and field curvature. Several possible routes presentthemselves as likely solutions to this problem. The possible solutionsare to,

(a) correct for aberrations at the recording stage by incorporatingcorrecting elements into the camera configuration,

(b) record the hologram in the normal manner and correct for aberrationsat the replay stage by incorporating correcting elements into thereconstruction configuration,

(c) record and replay without correction and correct by computermanipulation of output data, or,

(d) some combination of all three.

Of the above options it would seem that the most appropriate route is torecord the hologram in the normal manner and correct for aberrations atthe replay stage. The reasons for this being that the cameraconfiguration is kept as simple as possible, thereby keeping it smallerand more reliable, and more complex methods of correction can beemployed more readily in a laboratory based replay system.

A specific optic can be designed through which the hologram can bereplayed such that the aberrations are corrected.

An initial approach is to replay the hologram through a simple parallelplate such that aberrations introduced by recording the hologram throughan optical window are removed.

If the hologram can be replayed at a shorter wavelength than that atwhich it was recorded such that the wavelength ratio is the inverse ofthe refractive index ratio between the two media, then it may bepossible to remove astigmatism caused by recording in water.

The incorporation of a holographic optical element (HOE) into the replaystage is one means of achieving correction. The holographic opticalelement is then substituted for an equivalent element made out of glass.

An alternative embodiment relies on replaying the hologram back througha replica of the distorting medium so that aberrations are cancelledresulting in a distortion-free image.

A possible reconstruction facility is shown diagrammatically in FIG. 20.This comprises an argon-ion laser 70 pumping a dye laser 71. The laserbeam 72 is directed by way of prisms 73,74 and collimating optics 75 toilluminate the rear of a hologram 76. A pseudoscopic real image 77 iscreated. Aberrations are corrected by means of an aberration correctingelement 78.

I claim:
 1. A holographic recording and reproducing systemcomprising:means for using a beam of radiation from a source forrecording holographic images of an object positioned in a first mediumand replaying said images in a second medium; means selectivelyresponsive to the polarization of radiation reflected from said source;and gating means for selectively limiting an extent, in a direction ofpropagation of said radiation, of the region from which images arerecorded substantially to reduce the effect on a photosensitive mediumof radiation other than radiation capable of producing said holographicimages, wherein said gating means includes coherence gating meansselectively to receive radiation from a predetermined target region. 2.A holographic recording and reproducing system as claimed in claim 1wherein said gating means includes range gating means selectively toreceive radiation from a predetermined target region.
 3. A holographicrecording and reproducing system as claimed in any one of the precedingclaims further comprising narrowband filter means selectively to passradiation from said source to said recording means.
 4. A holographicrecording and reproducing system as claimed in claim 1, furthercomprising a source of coherent radiation adapted to operate in thegreen region of the visible spectrum.
 5. A holographic recording andreproducing system as claimed in claim 4, wherein the source of coherentradiation is a frequency-doubled Nd-YAG laser.